1. Field
The present disclosure relates generally to inert gas welding. More particularly, the invention is directed to purge dams for retaining purge gas around a weld zone.
2. Description of the Prior Art
By way of background, inert gas welding is a species of arc welding in which the molten weld pool is shielded from atmospheric contamination and oxidation by bathing it with an inert gas, such as Argon, or a mixture of Helium and Argon. Popular examples of inert gas welding include TIG (Tungsten Inert Gas) welding and MIG (Metal Inert Gas) welding.
When welding together pipes and other enclosed structures using inert gas welding, it is important to purge the interior of the pipe or structure in the vicinity of the weld zone to prevent corrosion and the formation of oxides on the interior side of the weld pool. Purge dams are conventionally used for this purpose. For example, when butt-welding the ends of two pipe sections to form a consolidated pipe run, two purge dam structures are placed in the pipes, one in each pipe on either side of the weld zone. The inert purge gas can then be introduced into the area between the dams. During initial purging prior to welding, the introduction of inert gas into the weld zone displaces the ambient air therein until the oxygen content is reduced to an acceptably low level (as determined by an oxygen monitor). Following initial purging, the flow of inert gas is continued to maintain a pressure differential that prevents oxygen from re-entering the weld zone during welding.
Water degradable purge dams have been proposed that are made from water degradable paper. The advantage of water degradable paper purge dams is that they can be placed in close proximity to a weld zone, and then removed following welding by dissolving or otherwise degrading them with water introduced through the pipe. Insofar as pipe systems typically undergo hydrostatic pressure testing or flushing with water prior to use, water soluble purge paper can be used in many cases without any additional processing steps following welding.
When welding together pipe sections, inert gas is typically introduced into the weld zone via a purge gas dispensing tip that is inserted through the welding root gap. Alternatively, the purge gas dispensing tip can be inserted through a small hole in one of the purge dams. A disadvantage of these techniques is that the inert gas creates turbulence in the weld zone as it is discharged under from the dispensing tip under pressure. During the initial purge operation, this causes the purge gas to mix with the ambient air initially present in the weld zone, thereby increasing the time required to reduce the oxygen content of the weld zone to the desired level. Following initial purging, inert gas turbulence can result in instability of the welding arc as the weld is being formed.
Applicant submits that there is presently a need for improvement in the construction of water-degradable purge dams. What is required in particular is a degradable purge dam apparatus that reduces inert gas turbulence in a weld zone.